The effect of indomethacin on the synthesis of collagen, non-collagen proteins and on bone resorption was studied in a bone organ culture system, using calvarial bones from 6-day-old mice. It was found that indomethacin reduced the release of calcium, inorganic phosphate and hydroxyproline without affecting the total amount of hydroxyproline in the cultures. Indomethacin had no effect on the synthesis of non-collagen proteins as estimated by the uptake of 3H-tryptophan. The inhibition of the release of hydroxyproline and minerals could be seen in concentrations of indomethacin from 10(-5) to 10(-8) M. Prostaglandin E2 (10(-7) M) could prevent the blocking capacity of indomethacin (10(-6) M), suggesting that the inhibitory action of the drug was due to the inhibition of endogenous prostaglandin synthesis. The results presented are compatible with the concept that indomethacin causes the reported detrimental skeletal effects by means of reduced osteoclastic rather than by reduced osteoblastic activity.